A conventional method for stabilizing oscillation frequency separation among plural laser devices has been described on page 2-204 of "Part 2, National Conference Record, 1986, The Institute of Electronics and Communication Engineers of Japan". In the method for stabilizing oscillation frequency separation among plural laser devices, an oscillation frequency of a laser device selected from the plural laser devices is stabilized to be based on a resonant frequency of a Fabry-Perot etalon. Oscillation frequencies of the remaining laser devices which are observed through a scanning Fabry-Perot optical resonator are stabilized to be provided with a predetermined separation based on the oscillation frequency of the selected laser device.
A further conventional method for stabilizing oscillation frequency separation among plural laser devices has been described on pages 61 to 64 of "Technical digest, Vol.III, 5th International Conference on Integrated Optics and Optical Fiber Communication, Oct. 1 to 4, 1985". In the method for stabilizing oscillation frequency separation among plural laser devices, the plural laser devices in which an oscillation frequency of a selected laser device is stabilized are to radiate light outputs with respective frequencies, while an oscillation frequency sweep laser device is driven to radiate a frequency swept light output in accordance with a sawtooth sweep signal. The light outputs radiated from the plural laser devices are coupled with the frequency swept light output radiated from the oscillation frequency sweep laser device to produce beat signals which are then converted to a train of pulse signals. The plural laser devices are controlled, such that generation times of the pulse signals are of respective predetermined separations as compared to a generation time of a pulse signal which is produced on the basis of light output radiated from the selected laser device having a stabilized oscillation frequency. As a result, the plural laser devices are stabilized to radiate light outputs among which oscillation frequencies are maintained to have a predetermined separation.
A still further conventional method for stabilizing oscillation frequency separation among plural laser devices has been described in "The Technical Research Report, Vol. 87, CS87-96, 1987, The Institute of Electronics, Information and Communication Engineers of Japan." In the method for stabilizing oscillation frequency separation among plural laser devices, an optical resonator is introduced to a control system of the second method as described above in which the optical resonator is used for a reference of the oscillation frequency separation.
However, the following disadvantages are observed in the first to third methods as described above. In the first method, the Fabry-Perot optical resonator is used for realizing the oscillation frequency separation. As a result, the number of laser devices which are simultaneously controlled in the oscillation frequency separation is limited to approximately 10 to 20, because the number depends on a finesse of the Fabry-Perot resonator. Further, the mechanical stability is low, because the mechanical sweeping operation is carried out. In the second method, a reference of the oscillation frequency separation must be brought into a system for performing this method, because the reference of the oscillation frequency separation is not provided in the system. Further, the reference of the oscillation frequency separation is determined by a relation between the pulse generation times obtained from the beat signals and voltages applied to the laser devices at the times, which is based on a relation of a swept oscillation frequency changed relative to a voltage applied to the oscillation frequency sweep type laser device, so that the oscillation frequency separation initially set is not maintained, where the relation changes due to the secular variation of the laser devices, and a target voltage must be set again in a laser device which is newly included in place of a laser device of a fault. In the third method, it is inevitable that an oscillation frequency deviates even in a laser device operating correctly, where any fault occurs in some of the laser devices in which the oscillation frequency separation is controlled, or in the laser device in which an oscillation frequency is swept, although the disadvantages of the first and second methods are overcome.